This invention relates to a hydraulic pump that can be used as the pressure generator source in a pressure fluid device; more specifically, it relates to a hydraulic pump that can be used in an antilock brake system (ABS) or traction control system (TCS).
Conventional hydraulic pumps are of a variety of designs. For example, the pump disclosed in the Publication of Unexamined German Patent Application Number 3 236 536 is configured from a piston guide housed inside a housing cavity wherein a piston slides back and forth within the piston guide. This piston slides in one direction upon receipt of a drive force from the eccentric cam of the motor's drive shaft, then slides in the reverse direction upon receipt of the spring force of a piston return spring.
This hydraulic pump also includes a suction check valve and an exhaust check valve. When the piston backs up towards the eccentric cam by the spring force of the piston return spring, the exhaust check valve closes the fluid outlet line, wherein the suction check valve opens the fluid inlet line and sucks fluid into the pump chamber from the inlet line side.
A conventional hydraulic pump uses a mechanical sealing configuration around the outer circumference of the piston to seal that area between the pump chamber and the suction line side. A microscopic gap of the order of several microns occurs between the piston and the piston bore. This type of mechanical seal presents some problems as follows.
Components must be fabricated with very high precision. Moreover at high temperatures, the viscosity of the pressure fluid (for example brake fluid) decreases, causing a larger amount of fluid to leak out of the pump chamber and back into the suction line, which reduces the delivery capacity of the pump.
In using a mechanical seal configuration, since the piston is made from a very hard material such as a quenched steel, in order to prevent wear of the piston bore in which the piston slides, the piston guide which is made from a very hard material like steel must be interposed. This extra component adds to the cost of parts and to the cost of assembly.
Deviations from the manufacturing tolerances are unavoidable in the fabrication of the piston and piston bore. Selective matching of a piston to a piston bore of proper dimension to assure a clearance of several microns is required and is time and labor-consuming.
In a normal hydraulic pump, the space between the pump chamber and exit chamber is sealed by cladding two inelastic flat surfaces (the face of the piston guide and the face of the valve unit). Accordingly, the large contacted area reduces the surface pressure per unit of area of the contacted area, making it difficult to achieve a tight seal. As well, sealing effectiveness will be easily reduced by distortions and scratches in the contacted joint.